As a result of the constantly increasing integration density in the semiconductor industry, photolithographic masks have to project smaller and smaller structures onto a photosensitive layer, e.g. a photoresist on wafers. In order to fulfil this demand, the exposure wavelength of photolithographic masks has been shifted from the near ultraviolet across the mean ultraviolet into the far ultraviolet region of the electromagnetic spectrum. Presently, a wavelength of 193 nm is typically used for the exposure of the photoresist on wafers. As a consequence, the manufacturing of photolithographic masks with increasing resolution is becoming more and more complex, and thus more and more expensive as well. In the future, photolithographic masks will use significantly smaller wavelengths in the extreme ultraviolet (EUV) wavelength range of the electromagnetic spectrum (e.g. in the range of 10 nm-15 nm).
Photolithographic masks have to fulfil highest demands with respect to transmission homogeneity, planarity, pureness and temperature stability. The tolerable deviation of their substrates from the planarity is only a portion of a wavelength of the exposure wavelength in order to not significantly disturb the phase front of the electromagnetic wave reflected from a multi-layer structure on a surface of the substrate. Larger deviations of the planarity of the substrate of the photolithographic mask may lead to variations of the optical intensity distribution in the photoresist due to a constructive or a destructive superposition of the wave front in the photoresist. A decrease of the exposure wavelength makes this problem more challenging. The substrate as supplied from the manufacturer may not even fulfil the planarity condition for EUV photolithographic masks and the manufacturing process of the mask which forms fine patterns on one surface may even deteriorate the planarity of the substrate.
The US patent application US 2007/02245222 A1 describes a method to improve the planarity of a manufactured photolithographic mask. The U.S. Pat. No. 7,001,697 B2 provides another method to eliminate intensity differences or optical transmission errors introduced by a photolithographic mask in the photoresist on a wafer.
For transmissive photolithographic masks the homogeneity of the optical transmission across the mask area is an important parameter. The uniformity of a structure element across the area of the photolithographic mask is called critical dimension uniformity (CDU).
Moreover, photolithographic masks may also have placement errors of pattern elements, i.e. some of the pattern elements do not image the pattern parameters exactly at the predetermined position on the photoresist. Placement errors are also called registration errors.
The action of femtosecond laser pulses on a quartz substrate has for example been investigated by S. Oshenkov, V. Dmitriev, E. Zait, and G. Ben-Zvi: “DUV attenuation structures in fused silica induced ultra-short laser radiation”, Proc. CLEOE-IQEC, Munich 2007.
For an improvement of a registration and/or of a CD correction result, it is extremely important to control all parameters of the laser pulses in order to control the local deformation introduced in a substrate of a photolithographic mask.
On the other hand, it is clear that for a successful simultaneous CD and registration correction, it is necessary that the laser pulses are able to induce any possible elementary deformation in the mask substrate. For this purpose, it is required to provide a comprehensive set of laser pulses leading to a broad spectrum of local deformations. This spectrum comprises at the one end laser pulses which provide a minimum local deformation at a given attenuation of the optical transmission in order to correct CD errors. On the other hand, in order to correct placement or registration errors, laser pulses are required having a large asymmetric local deformation.
It is therefore one object of the present invention to provide a method and an apparatus for locally deforming an optical element and to avoid at least a portion of the problems discussed above.